A vehicular occupant protection system is proposed, for instance, in JP-H8-67231A (US005544915A). Here, the system includes a center acceleration sensor located in a center of the vehicle and left and right sidewall acceleration sensors located around center pillars of the left and right sidewalls of the vehicle. The center acceleration sensor detects a collision of an obstruction with the front or side of the vehicle; the left and right sidewall acceleration sensors, with the left and right sidewalls, respectively.
A collision of the vehicle is determined when the center acceleration sensor detects a sensing level exceeding the highest (largest) threshold of multiple thresholds, or when the center acceleration sensor detects a sensing level exceeding a threshold lower (smaller) than the highest and one of the sidewall acceleration sensors detects a sensing level exceeding a given threshold. Determining of the collision activates the occupant protection system.
Further, another occupant protection system is proposed in JP-H11-180249 (US006005479A) by the same applicant as that of this application. Here, collision of a vehicle is determined by using a phase difference in detected acceleration between a sidewall and center acceleration sensors. In detail, a collision pattern is determined by estimating an equivalent of an intrusion amount of a vehicle as an obstruction, using a phase (or time) difference in integration output of detected acceleration between the sidewall and center acceleration sensors. The phase difference is determined by a time difference between when integration output of the center acceleration sensor exceeds a threshold and when integration output of the sidewall acceleration sensor exceeds a threshold.